The difference between Marketing Leads and Sales Leads is that sales has made contact with the lead and determined the lead's readiness to buy , whereas in case of Marketing Lead it is just that there is a Lead that has shown interest because of the Marketing activity done.

Now for this Marketing lead to become a Potential Sales Lead there needs a Confirmation activity via a Visit ,Phone call to gain potential .

It takes good old-fashioned human-to-human contact to determine whether a lead is really interested in becoming a customer